A conventional boiling water reactor is shown in FIG. 1. Feedwater is admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12 and a feedwater sparger 14, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. The feedwater from sparger 14 flows downwardly through the downcomer annulus 16, which is an annular region between RPV 10 and core shroud 18. In addition, a core spray inlet 11 supplies water to a core spray sparger 13 via core spray piping 15.
Core shroud 18 is a stainless steel cylinder surrounding the nuclear fuel core 20. Core 20 is made up of a plurality of fuel bundle assemblies 22 (only two 2.times.2 arrays of which are shown in FIG. 1). Each array of fuel bundle assemblies is supported at the top by a top guide 19 and at the bottom by a core plate 21. The core top guide provides lateral support for the top of the fuel assemblies and maintains the correct fuel channel spacing to permit control rod insertion.
The water flows through downcomer annulus 16 to the core lower plenum 24. The water subsequently enters the fuel assemblies 22, wherein a boiling boundary layer is established. A mixture of water and steam enters core upper plenum 26 under shroud head 28. Vertical standpipes 30 atop shroud head 28 are in fluid communication with core upper plenum 26. The steam-water mixture flows through standpipes 30 and enters steam separators 32, which are of the axial-flow centrifugal type. The separated liquid water then mixes with feedwater in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus. The steam passes through steam dryers 34 and enters steam dome 36. The steam is conducted from the RPV via steam outlet 38.
The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation water outlet 42 and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 44 (only one of which is shown) via recirculation water inlets 46. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The jet pump assemblies are circumferentially distributed around the core shroud 18.
The core shroud 18 (see FIG. 2) comprises a shroud flange 18a for supporting the shroud head 28; a circular cylindrical upper shroud wall 18b having a top end welded to shroud flange 18a; an annular top guide support ring 18c welded to the bottom end of upper shroud wall 18b; a circular cylindrical middle shroud wall comprising three sections 18d, 18e and 18f welded in series, with a top end of section 18d being welded to top guide support ring 18c; and an annular core plate support ring 18g welded to the bottom end of middle shroud wall section 18f and to the top end of a lower shroud wall 18h. The entire shroud is supported by a shroud support 50, which is welded to the bottom of lower shroud wall 18h, and by annular shroud support plate 52, which is welded at its inner diameter to shroud support 50 and at its outer diameter to RPV 10.
In the event of a seismic disturbance, it is conceivable that the ground motion will be translated into lateral deflection relative to the reactor pressure vessel of those portions of the shroud located at elevations above shroud support plate 52. Such deflections would normally be limited by acceptably low stresses on the shroud and its weldments. However, if the shroud weld zones have failed due to stress corrosion cracking, there is the risk of misalignment and damage to the core and the control rod components, which would adversely affect control rod insertion and safe shutdown.
Stress corrosion cracking in the heat affected zone of any shroud girth seam welds diminishes the structural integrity of shroud 18, which vertically and horizontally supports core top guide 19 and shroud head 28. In particular, a cracked shroud increases the risks posed by a loss-of-coolant accident (LOCA). During a LOCA, the loss of coolant from the reactor pressure vessel produces a loss of pressure above the shroud head 28 and an increase in pressure inside the shroud, i.e., underneath the shroud head. The result is an increased lifting force on the shroud head and on the upper portions of the shroud to which the shroud head is bolted. If the core shroud has fully cracked girth welds, the lifting forces produced during a LOCA could cause the shroud to separate along the areas of cracking, producing undesirable leaking of reactor coolant.
A known repair method for vertically restraining a weakened core shroud utilizes tensioned tie rods coupled to the shroud flange and to the shroud support plate. In addition, the shroud is restrained laterally by installation of wishbone springs which, along with the tie rod, are components of the shroud repair assembly. During repair of the shroud, the respective components of the repair assembly must be lowered into the annulus for installation in sequence. This installation of hardware in turn requires various tooling to be lowered into the annulus and later removed via cables. The feedwater sparger 14 and core spray piping 15 (best seen in FIG. 3) are located above and partially obstruct the vertical access to the annulus 16. To protect the feedwater sparger and core spray piping against damage during the lowering of various hardware and tooling into the annulus, a cover is installed over the feedwater sparger and core spray piping at the azimuthal location of the hoist. This cover further limits the vertical access to the annulus. If left unprotected, the steam dam 29 could be damaged by hardware and tooling as attempts are made to enter the annulus while avoiding the feedwater sparger and core spray piping. Thus, there is a need for a tool which will protect the steam dam against damage, which will guide lowered hardware or tooling into the annulus, and which can manipulate the hoisting cable to move the hardware or tooling suspended at the end of the cable in a radially outward direction to a desired position in the annulus.